This invention relates generally to holddown lock screw assemblies as are commonly used in pressurized wellhead structures for such functions as energizing a wellhead seal and more particularly to a holddown lock screw assembly for energizing a dual crossover seal assembly where balanced energizing loads are required.
Holddown screws are used in many applications to provide a preload for energizing wellhead seals. Typically, the nose end of each of a plurality of screws enegages a camming surface of the seal device to apply an energizing force in either the upward or downward direction depending upon the particular type of seal device. The nose end of the holddown screw is customarily of conical configuration and is adapted to engage a frusto-conical camming surface of the seal device to be energized. On occasion, a holddown screw is used to simultaneously apply an energizing force in both the upward and downward directions as when applying a preload to the dual sealing devices of a dual crossover seal assembly. Such an installation is shown in U.S. Pat. No. 4,556,224.
A crossover seal is a packing mechanism designed to be used in the lower flanges of a casing head and/or tubing head to provide a seal between those heads and the portion of casing extending therein which is being suspended in the head below. A crossover seal enables the head in which it is installed to be rated to the next higher pressure rating above that of the lower flange, provided that the upper flange, and outlets of that head are of the same higher pressure rating. Dual crossover seal assemblies are oftentimes provided for the sake of redundancy and safety. However, when disposed in tandem array as shown in U.S. Pat. No. 4,556,224, FIG. 2 thereof, it is extremely difficult to obtain a balanced loading of both seal assemblies because of manufacturing tolerances for the various parts of the seal assemblies and the holddown screws. Accordingly, testing of such dual assemblies is unreliable and true redundancy and safety is not achieved. Also, while it is known to provide a holddown screw with a separately mounted nose piece, as shown in U.S. patent application, Ser. No. 06/845,912, entitled HOLDDOWN SCREW and filed Mar. 31, 1986, the purpose of such a nose piece is to reduce or minimize galling between the contacting surfaces of the holddown screw and seal assembly and it is incapable of adjusting for tolerance error stock-ups to apply balance loading to a dual crossover seal assembly.
Accordingly, it is therefore an object of the invention to provide a loading mechanism which can be actuated by screws to apply a balanced preload to both sealing devices of a dual crossover seal assembly.
It is another object to provide a segmented loading ring assembly which can be actuated by holddown screws to apply a balanced preload substantially uniformly about the circumference of the annular sealing devices of a dual crossover seal assembly in a pressurized wellhead.